The above-mentioned hand-held small work apparatuses include a brush cutter used in mowing, chain saw used in logging or trimming, and a hedge trimmer used in hedge trimming, all of which are well-known. The brush cutter, chain saw, etc. use a cutting blade that moves at a high speed. For this reason, there have been developed work apparatuses provided with a safety device intended to protect the worker using the work apparatus from the cutting blade in order to assure his or her safety at occurrence of an unexpected situation during the work.
Japanese Utility Model Laid-open Publication No. H02(1990)-131822 (hereafter referred to as “Patent Document 1” as well) discloses a shoulder-strap type brush cutter. On each of right and left handles of the brush cutter, a stop lever is provided. The stop lever is actuated when grasped by the worker. When the worker takes his/her hand off at least one of the right and left stop levers, namely, when the stop lever is released, a fuel valve mechanically linked to the stop lever is closed, and forcibly stops fuel supply to the engine or supply of power to an ignition plug of the engine to thereby stop the engine from running.
Japanese Patent Laid-open Publication No. H04(1992)-158714 (hereafter referred to as “Patent Document 2” as well) discloses a shoulder-strap type brush cutter in which a stop lever or pressure sensor is provided on each handle of the brush cutter to actuate an electromagnetic brake or mechanical stopper to thereby forcibly stop the cutting blade from rotating when the worker takes the hand off the handle.
Japanese Patent Laid-open Publication No. 2006-288296 (hereafter referred to as “Patent Document 3” as well) discloses a hand-held type electric motor-driven brush cutter having a shock sensor. The shock sensor includes a piezoelectric element that abuts a manipulation tube provided around a power transmission shaft connecting an electric motor as a drive source and a cutting blade to each other. When the shock sensor detects a shock, the motor is forcibly stopped. In this Patent Document 3, it is also proposed to provide, in the safety device including the shock sensor, a variable resistor for adjusting a current flowing through the shock sensor to eliminate the variation in sensitivity from one piezoelectric element to another.
Japanese Patent Laid-open Publication No. H08(1996)-187024 (hereafter referred to as “Patent Document 4” as well) discloses a shoulder-strap type brush cutter including an internal combustion engine or electric motor as a drive source and a centrifugal clutch interposed between the drive source and a power transmission shaft that mechanically connects the drive source and a cutting blade. The centrifugal clutch includes a clutch shoe that is mechanically activated by large relative displacement, if any, between the drive source and the power transmission shaft to automatically brake the centrifugal clutch.
Japanese Patent Laid-open Publication No. 2008-118960 (hereafter referred to as “Patent Document 5” as well) discloses a shoulder-strap type brush cutter. The strap for a worker to carry the brush cutter on his/her shoulder has a cap at its distal end, and it can detachably engage with a switch main body of the brush cutter. In this shoulder-strap type brush cutter, if the cutting blade hits a large stone or the like and the brush cutter is kicked back, the cap disengages from the switch body, and power supply to the ignition plug is forcibly interrupted to stop the engine.
Japanese Patent Laid-open Publication No. S62(1988)-198320 (hereafter referred to as “Patent Document 6” as well) proposes to provide a mercury switch that detects an angle of inclination of a manipulation tube in form of a sleeve covering a power transmission shaft that transmits the output of an electric motor to a cutting blade. In this proposal, once the detected inclination angle of the manipulation tube gets out of a predetermined range, power supply to the electric motor is interrupted to stop it forcibly.